Heat treatment process for non-alloyed low-carbon structural steel

ABSTRACT

A LOW CARBON STEEL CAN BE HEAT TREATED TO IMPROVE EITHER ITS PHYSICAL PROPERTIES OR ITS ELONGATION WITHOUT DETRIMENT TO THE OTHER BY HEATING A SHELL REGION OF THE STEEL QUICKLY AND THEN ALLOWING HEAT TRANSFER TO TAKE PLACE UNTILE THE SHELL TEMPERATURE PREFERABLY DROPS TO THE VALUE OF THE CORE TEMPERATURE WHEREUPON THE STEEL IS INTENSIVELY QUENCHED IN WATER.

March 5, 1974 TE RE ET AL 3,795,550

HEAT TREATMENT} PROCESS FOR NON-ALLOYED LOW-CARBON- S'IRUCTURAL STEELFiled April 29, 1971 Tns [Joli United States Patent O 3,795,550 HEATTREATMENT PROCESS FOR NON-ALLOYED LOW-CARBON STRUCTURAL STEEL LudwigEttenreich, Wien, Austria, and Otto Reimann, Dusseldorf-Oberkassel, andKlaus Greulich, Erkrath, near Dusseldorf, Germany, assignors tBau-Stahlgewebe G.m.b.H.

Filed Apr. 29, 1971, Ser. No. 138,657 Claims priority, applicationGermany, Apr. 30, 1970, P 20 21 245.7;Mar. 13, 1971,? 21 12 103.9

Int. Cl. C21d 1/18 US. Cl. 14812.4 19 Claims ABSTRACT OF THE DISCLOSUREA low carbon steel can be heat treated to improve either its physicalproperties or its elongation without detriment to the other by heating ashell region of the steel quickly and then allowing heat transfer totake place until the shell temperature preferably drops to the value ofthe core temperature whereupon the steel is intensively quenched inwater.

BRIEF SUMMARY OF THE INVENTION This invention relates to the heattreatment of nonalloyed low-carbon structural steel (max. 0.26% C) forimproving the physical properties thereof. The steel is in the form ofbars continuously passed through a rapidheating zone, and is heated to ahigh temperature and subsequently quenched.

The physical properties of non-alloyed steel depend essentially on thecarbon content (neglecting heat treatment or mechanical workingoperations). With increasing carbon content, the tensile strength andthe elastic limit increase, while the elongation decreases (cf.Werkstotfhandbuch Stahl un Eisen, 4th edition, 1965, G 1, in particularpictures 1 and 2). Increased strength properties are obtained byhardening and annealing in relationship to the carbon content.Generally, a minimum carbon content of more than 0.30% by weight isconsidered to be necessary for conducting this heat treatment fornonalloyed heat-treatable steel, but the weldability is thereby impairedor rendered impossible.

Attempts have been repeatedly made to develop processes in order toimprove the physical properties of nonhardenable steel.

Thus, for instance a process is known from Stahl und Eisen 1949, pages186-194, in which conventional strength-increasing alloy elements werereduced in structural steel of the type St 52 in order to save suchalloy elements. Therein steel with a carbon content in accord with St52, but with lesser contents of manganese and other strength-increasingelements, was increased in strength by quenching sheets in Water whichwere rolled and thereby heated.

For steel having a content of 0.18% C and 0.46% Mn, an increase instrength was obtained in the desired degree but with a concurrentdecrease in elongation. This process is not acceptable in practicebecause the values obtained were subject to high fluctuations.

In the late nineteen-fifties a heat treatment process originating fromlaboratory tests became known for low carbon steel wire, the steel wiresubsequently being further processed in cold condition. According tothis process, steel wires of 2-8 mm. in diameter were inductively heatedto high temperatures and subsequently quenched in water. With a risingquenching temperature, an increase in the tensile strength and adecrease in the elongation resulted. These laboratory tests have notresulted in a manufacturing-scale process.

ice

A further process for thorough annealing (hardening and tempering) ofarticles composed of non-hardenable steel is known from Austrian patentspecification 281,089, wherein steel articles having a carbon content ofabout 0.10 to 0.23% are treated in a manner similar to surface hardeningby means of gas/oxygen burners or electrical heating devices andsubsequent water sprays, the articles being advanced at a minimum rateof travel of about to 200 mms. per minute. The heat treatment results inan increase in hardness, the hardness diiferential between the surfaceand the core in the non-annealed condition being broadly balanced in thesubsequent annealing operation depending on the strength required, sothat reference can actually be had to an annealing (tempering)throughout.

Contrary to the cited prior art which exclusively results in theincrease of one specific material property at the sacrifice of another(e.g. increase of strength accompa nied by a decrease of elongation),the present invention seeks to provide a method in which one propertycan be improved While the others are at least retained but preferablyalso increased.

The solution of this objective is particularly applicable to reinforcingsteel for concrete, since thereby the safety of buildings can besubstantially increased. A structural steel mat having conventionalstrength properties, but a greatly increased elongation gives a muchgreater yieldability to the building along with increased safety. On theother hand, a great strength increase with conventional elongation ratesis particularly applicable for prestressed concrete reinforcement.

The objects of the invention are achieved by the following measures:

(1) The steel is heat-treated in a cold-deformed condition and (2) Isheated to a temperature of between 600 and 1300 C. only in acircumferential shell region; the total heat content introduced into theshell region being controlled by adjustable parameters of power densityand penetration depth (e.g. for induction heating: power density inw./cm. and frequency) such that the heat content serves to heat the coreat a rate of at least 100 C./sec. and preferably at least 300 C./sec. toraise the temperature of the core between 450 C. and Ac and (3) Theheated steel is quenched within a temperature balancing range (t t taccording to FIG. 1), the time initiation value (t-=t according toFIG. 1) at which the core temperature is 450 C.

Preferably, the quenching operation is effected with water, particularlyin the form of water jets.

The degree of cold deformation of the treated steel is between 10 to70%, and preferably between 20 and 45%.

Preferably, the described heat treatment is conducted for steel having acarbon content of 0.06 to 0.26% carbon, and particularly 0.08 to 0.22%,the steel having conventional contents of silicon, manganese etc. oralloy contents as occur in so-called high-strength structural steel.

Preferably, the core is heated at an average rate of about 700 C./sec.The specified average value can be determined by dividing the desiredmaximum value of the core temperature by the total time required toachieve this (time from the introduction into the induction coil to thebeginning of the quenching operation).

The manganese content of the treated steel can be advantageouslyincreased from 0.8 to 1.8%.

When it is desired to increase the elongation properties (6 6 whilemaintaining or even increasing the strength properties, the shell regionof the steel is heated to at least 700 C., and for the temperaturebalancing range (t stsn according to FIG. 1) in the core a temperatureof at least 600 C. is produced while the core is at a maximum of 750 C.and no more than a value slightly above Ar (t=t according to FIG. 1).Thereupon a quenching is effected at an average cooling rate of at least800 C./sec. The specified average value of the cooling rate relates tothe period of time in which the surface has been cooled to a temperaturebelow 150 C. At the begining of the quenching operation, a significantlyhigher value is obtained.

At the beginning of the quenching operation a cooling rate between 1200and 1500 C./sec. is preferred.

The upper limit of the process of this invention is characterized inthat prior to reduction of the temperature of the shell region (t=taccording to FIG. 1) to a value .below 550 C., the quenching iseffected.

Of the conventional cooling fluids, a spray treatment with water at apressure of 3 to 5 atmospheres is preferred. In this treatment, Water inan amount between 6 and liters per kg. steel and preferably between 6 to10 liters per kg. steel is used. The weight of steel corresponds to theamount of bar steel conveyed into the quenching zone.

At the beginning of the quenching operation, the quenching is preferablyeffected at a cooling rate of 1450 to l700 C./sec.

The quenching operation is effected before the temperature of the shellregion (t=t according to FIG. 1) is below 700 C.

Preferably, the quenching is effected at the higher cooling rate byspraying water on the steel member at a pressure of 7 to 12 atmospheres.The amount of water is at the rate of 10 to 30 liters per kg. steel, andpreferably between to 30 liters per kg. steel.

It is advantageous to subject the steel member to a slight colddeformation to the extent of a straightening, after the quenching forspecific uses.

For other purposes, particularly for the elevation of the elasticitylimit (as well as the creep strength), it is of advantage to effect aheat treatment at temperatures between 100 and 380 C. after quenching,preferably around 340 C., over a period of time in which the elasticlimit greatly increases. An example of such a retention time is 20 to 30minutes.

The bars treated according to this invention advantageously have adiameter of 4 to 36 mms., and preferably between 6 to 16 mms.

The heat treatment of this invention affords particular significance andadvantages in its use for cold deformed bars of structural steel, inparticular steel for prestressed or unstressed reinforcement members andparticularly bars for welded structural steel mats.

Within the specified analysis limits the silicon content is normallylimited to a maximum of 0.5%, the manganese content to a maximum of 0.8%

It can be advantageous to quench shortly before reaching the temperaturebalancing range (t gtst according to FIG. 1) with core temperatures inthe upper range of crystal recovery, preferably 450 to 550 C., and withshell surface temperatures above Ar for increasing the elongationproperties (5 6 While practically maintaining or even improving thestrength properties. The quenching of the shell surface region is thenpreferably effected to a temperature below the crystal recovery range,in par ticular to a temperature around 200 C. After the surface regionhas been coded to this temperature, the remaining temperature balancingis conducted in air, the quenched surface region readily recovering heatfrom the core region.

The particular advantage of the process of this invention is that, instarting with a low-carbon non-alloyed structural steel having aspecific cold deformation degree (e.g. 36%) which in this state hasspecific strength values and elongation values, improved properties canbe obtained by a differentiated heat treatment in which uniformannealing is not permitted over the entire bar crosssection. As comparedwith the initial values in the cold deformed state on the one hand steelhaving greatly increased elongation properties While maintaining or evenimproving the strength properties can be obtained whereas, on the otherhand, steel having greatly increased strength properties whilemaintaining the elongation properties can be produced. This result is incontradistinction t0 the present state of the art Where it is believedthat strength values and elongation values can only be individuallyincreased to the detriment of the other.

By the term cold deformation degree referred to above is meant any coldmechanical working to the extent of reduction of the cross section asindicated. Specifically a cold drawing of 36% is contemplated.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a graph showing variation oftemperature with respect to time for a shell region and a core region ofa steel member.

DETAILED DESCRIPTION In the outset, several terms used in thespecification and in the claims are to be defined:

The steel member is described with reference to an elongated member inthe form of a bar and such memher is intended to have the samecross-sectional shape and area in planes perpendicular to thelongitudinal axis of the member for the length of the member. The barsmay be circular or of any polygonal shape.

The bar is referred to as having a Shell region and this is understoodto be the circumferential layer surrounding a core region of the bar, Aswill be disclosed later, the thickness of the shell, i.e. moreparticularly the volume of the shell in relation to the volume of thecore, is determined by the desired core temperature. Technically, thethickness of the desired shell is determined by the frequency of aninductor coil as employed when heating by the induction method. Thethickness of the shell increases, the lower the frequency. The amount ofheat transmitted is controlled by the coil dimension and the powerdensity. The higher the power density, the quicker the shell heats andthe higher the temperature it reaches. Thus, by regulating the frequencyand the rate of advance of the bar relative to the coil dimension andthe powed density (retention time of the bar in the induction zone), adesired temperature in the shell is obtained. The relating of therequired power density to the ratio of shell volume to core volume iseffected according to the desired temperature of the temperaturebalancing range.

For a better understanding of the term temperature balancing rangereference is next made to FIG. 1.

In FIG. 1 there is diagrammatically shown the relationship oftemperature and time in the shell region and core region of the bar byan inductive heating without a subsequent quenching with Water and withcooling in air by radiation. The diagrammatic illustration shows theapproximate temperature variation of a wire having a diameter of 8 mm.which as a result of a specific frequency primarily has been heated to adepth (shell thickness) of about 0.8 mm. by controlling the frequency ofan induction source. As shown in the illustration, the core is alsoheated to a minor degree during the retention time in the inductioncoil.

In the illustrated example, the major part of the heat content in theshell region is transferred to the relatively cold core after the barleaves the induction coil (T in FIG. 1). This balance of heat over thebar cross-sec-- tion is effected during the temperature balancing rangeAt. For accomplishing the effects of this invention, the subsequentwater quenching must be effected Within a specific period of time in thecourse of the heat balanc ing operation between the shell region and thecore region.

The diagram shown in FIG. 1 shows the temperature variation withoutsubsequent water quenching and only the lower and the upper limits t and23 have been shownim which water quenching is to be effected. At thelower time limit t it is necessary that the core temperature be above T(450 C.). At the upper time limit (t the shell surface temperature isnot allowed to drop below T The temperature values specified refer tothe limit conditions in the shell region and in the core. It is clearthat for the extraordinarily quick heating and cooling phenomena, thedifferential temperature values between the shell surface and the corecan differ therefrom, i.e. for instance are higher at the time t=t Theprocess of this invention does provide a clear teaching, however of thelimit conditions.

When following the teaching of this invention the subsequent aspectsmust be further considered:

A deviation can result between the temperature curves and these curvescan be simulated by means of computers representing the shell and thecore temperature relative to the actual temperatures obtained inpractice because in the computer program the radiation losses from thesurface to atmosphere after leaving the induction coil have not beenconsidered. On the other hand, it is a simple matter for one skilled inthe art to determine the surface temperature by means of a disappearingfilament pyrometer. For this reason the temperature values specified forthe shell region which characterize the upper time limit (t;,) of thetemperature balancing range pertain to measured pyrometer temperatures.It is added that the upper time limit of the temperature balancing rangeis not preferred in practice and serves to limit the maximum timeinterval for the water quenching operation. In practice quenching ispreferably effected Within the specified temperature balancing rangewhen the increasing core temperature and the decreasing surfacetemperature intersect each other.

The time of this intersection can readily be determined in practice bymeans of a pyrometer measurement, since after the heat balancing of thecore, the temperature drop at the shell surface is much reduced, as heatemission is only effected further by radiation to the atmosphere.

There will next be explained in detail the basis for establishing thelimit conditions of the process of the invention.

The shell is heated to a temperature between 600 and 1300" C. with thecondition that the heat content introduced into the shell is sufficientto heat the core at an average rate of at least 100 C./sec. andpreferably at least 300 C./sec., to a temperature between 450 C. and AcAs disclosed by the following examples, a core temperature of 700 C. ispreferred. It is calculated from the preferred heating rate of anaverage of 300 0/ sec. and a core temperature of 700 C. that 2.33seconds elapse from the time the bar enters the coil induction to thetime of the water quenching operation. Starting from the specified limitconditions and a bar diameter of 8 mm., it can be calculated that for aretention time of 1.3 sec. in the induction coil, a frequency of 485kc.p.s. and a power density of 850 w./cm. are required. These conditionscan just as well be accomplished with a lesser frequency (to provide agreater shell thickness) and a lesser power density. An alternative isto heat a thin shell region to a high temperature, the core stillremaining relatively cold.

According to this invention, this short-time heating is effected only inthe surface region or shell region. Subsequently, the heat contentstored in the shell is emitted into the air, while the major portion istransmitted toward the core region. In this regard, the heat contentstored in the surface region for the purpose of obtaining an increase inthe elongation properties must be sufficient to heat the core at leastto a temperature in the upper crystal recovery range (no structuralvariations yet), and preferably at least 450 to 550 C. (FIG. 1, abscissat The temperature thereat still initially remains above Ar in thesurface region. In the following quenching operation with water, thesurface region is cooled to temperatures below 200 C., while theremaining temperature balance can be effected in air, the quenchedsurface region receiving the heat emitted from the core region.

As shown by the following first example, an increase in the tensilestrength is obtained with a great increase of elongation in steeltreated according to this invention. Table 1 shows the initial valuesand the final values achieved. Column 1 shows the temperature to whichthe surface region (shell) has been heated over a short time.

TABLE 1 [Analysis: Non-alloyed, low-carbon steel with 0.126) carbon,diameter 6.5 mm. in the cold deformed condition] Kp./mm. ElongationHeating of the surface region to C. 0'9, 0'5, rsk, percent Startingmaterial 35% deformation degree 0 55.0 5.0 8

As shown in the table, the greatest increase in the elongation is from 8to 15% with an increase in the tensile strength from 60 kg./mm. to 67.7kg./mm. when the surface region is heated to 900 C.

As alluded to initially, the time of the quenching is selected at whichafter heat balancing the temperature of the shell surface drops belowthat of the core (t=t This time can easily be determined in practice bya pyrometer measurement as previously described. For a specific bardiameter, this value can be established by the correlation of theadvance rate, frequency and power density within the limits specified bymeans of various procedures. Thus, for instance, a very thin shell (ascompared with the core volume) can be heated to a very high tecperaturein the induction coil (far above Ac or a very thick shell can be heatedto a temperature which is only 50 to 200 C. from the temperature at thetime t;, (according to FIG. 1).

TABLE 2 K mm. Pe nt T Q. (for t t2 g/ rce Vickers hardness m Fig. 1) anas 610 6; Surface Core A, 36% cold-deformation.- 61. 4 55. 0 8. 3 4. 0260 260 650 66.0 58.6 13.7 7.3 240 250 .0 51. 5 15.2 13.0 220 250 0 42.0 10. 0 5. 7 210 260 TABLE 3 Kg./mm. Percent Vickers hardness T 0.(t=tz) an as 610 5g Surface Core A, 36% co1d-deformation. 72 6. 9 2. 9273 266 600 72 62 ll. 8 6. 8 268 257 72 61 13. 5 9. 0 271 277 76 58 10.3 7. 3 315 274 78 56 8. 3 7. 3 423 273 TABLE 4 KgJmm. Percent Vickershardness T C. (t=tz) (TB as 51 5,; Surface Core Hereinafter, threefurther examples will be described with reference to the results inTables 2, 3 and 4. Tables 2, 3 and 4 respectively show the variations inthe physical material properties after the present heat treatment fordifferent bar cross-sections (6, 8 and 12 mms. 0). The quenching wasinitiated on reaching the moment of time within the temperaturebalancing range (FIG. 1t t t at which the increasing core temperatureintersect the decreasing shell surface temperature (FIG. 1t=t The firstlinevalue Aof the Tables 2, 3 and 4 represents in Retention time in thecoil sec 1 Frequency k.c.p.s 485 Total period of time to beginning ofquenching s c 2 Retention time in the cooling zone sec 0.5

For quenching two different intensities were selected (water quenching):

(I) 4 atmospheres (56.9 p.s.i.); quantity 8 litres/kg. steel (II) 8atmospheres (113.8 p.s.i.); quantity 26 litres/kg.

steel The bar still had a temperature of about 60 C. at the surfaceshortly after leaving the cooling zone for the quenching intensity (I).The cooling conditions listed under (II) belong to the range of intensecooling conditions. This range is characterized by spraying water at apressure of 7 to 12 atmospheres in amounts from 10 to 30, preferably 20to 30 litres per kg. steel. The quenching is effected preferably priorto a reduction in the decreasing shell surface temperature (l=taccording to FIG. 1) below 700 C. Under these conditions the coolingrate effected has an average value of at least 850 C./s. A typical,preferred cooling rate lies between 1450 and 1700 C.

As shown in Table 2, a substantial increase in the elongation propertiesis obtained with accompanying satisfactory strength values as comparedwith the cold deformed starting condition A for the lesser quenchingintensity I for a selected quenching temperature of 700 C. But evenquenching temperatures respectively 50 C. higher or lower still bringabout physical values which are far above those previously known.

It is to be now noted that for steel bars having larger cross-sectionswhich will be described later the preferred value of the temperaturebalancing range for the time t=t is lower, i.e. is 650 C. for barcross-sections between 8 and 12 mm. (700 C. for 6 mm.).

If following the heat treatment of this invention, a subsequent heattreatment is conducted at 340 C. for 30 minutes, increases of theelastic limit of up to 50% are achieved.

An increase of the carbon content within the described limits requires ahigher core temperature within the balancing range for maintaining theoptimum conditions as compared with the case of a reduction of thecarbon content, and vice versa. The temperature in the heated shell isadapted to the desired higher or lower core temperature.

In contradistinction to the prior art, the heat treatment process ofthis invention is successful in overcoming the conventional conceptionof antagonistic response of strength properties and elongationproperties for cold deformed starting material. This surprising fact canpossibly be explained when considering the Vickers hardeness values(Table 2) and associated textures (not shown). For the preferredimprovement of the elongation properties sought according to thisinvention, the hardness values for the surface region of the bar aresubstantially below the values of the core. It is possible that justthis distortion caused by the different structure in the surface and inthe core region brings about the properties of this invention. Theessential requirement is, however, that when employing the process ofthis invention, there is no uniform annealing. The core is to notundergo any phase conversion (u-y-u).

A bar of a diameter of 8 mm. (Table 3) shows just as good increases asthe bar with diameter of 6 mm. The core was heated at an average rate inthe range between 350 C./sec. and 420 C./sec. to the quenching time. Thecooling rate from the beginning of entering the water spray to the endof the water spray was about 900 C./ sec. This corresponds to coolingintensity (II). This cooling intensity also brought about good resultsfor a preferred alteration of the elongation properties. As shown inFIG. 3, the different hardness values of the shell and the core resultin the high strength increases with accompanying good toughnessproperties.

Table 4 shows that for larger bars (12 mm.) the differences in thehardness are not so marked.

Based on micrographic study, the deformation texture in the core remainsclearly visible.

The specific advantages of the bars made according to this invention arethat for bars having high elongation properties, structures withunstressed reinforcement are given a much higher safety margin as aresult of the high proportion of proportionality elongation, while thehighstrength bars are particularly suited for prestressed reinforcementmembers.

It is also notable that for low-carbon steels, the material propertiesachieved with this invention are not impaired, for instance, byresistance spot welding.

Examinations have been conducted at the joints of a welded structuralsteel mat. The evaluation of the minimum shear force in a shear test(S=0.3 X0 x cross-sec tional area of the starting bar) showed that thesteel mat satisfies the required minimum values.

What is claimed is:

1. A continuous heat treatment process for low-carbon steel (max. 0.26%C.) in bar-form being passed through a quick-heating zone to be heatedto a high temperature and subsequently being quenched, in which thesteel is heated in a 10 to 70% cold-deformed condition only in its shellto a temperature of between 600 and 1300 C. at a rate in which the coreis heated at an average of at least C./sec., to a temperature within atemperature range from 450 C. to Ac and the heated steel is quenchedduring the temperature balancing between core and shell without the corehaving undergone any phase transformation (ony-u) quenching being at theearliest started when the core temperature reaches 450 C. and at thelatent prior to reduction of the shell temperature to a value below 550C.

2. A process as claimed in claim 1 wherein said rate of temperatureincrease is a least 300 C./ sec.

3. A process as claimed in claim 1 wherein the steel has a carboncontent of 0.06 to 0.26% and silicon and manganese.

4. A process as claimed in claim 3 wherein the manganese content isbetween 0.8 and 1.8%.

5. A process as claimed in claim 1 wherein the degree ofcold-deformation is 20 to 45% 6. A process as claimed in claim 1 whereinthe steel member is heated to at least 700 C. in its shell region andthe core reaches a temperature of between 600 C. and 750 C. beforequenching, said quenching being effected at an average cooling rate ofat least 800 C./sec. whereby the elongation properties of the steelmember are increased while the strength properties are at leastsubstantially maintained.

7. A process as claimed in claim 6 wherein the cooling rate is between1200 and 1500 C./ sec.

8. A process as claimed in claim 6 wherein the shell surface temperatureis at least 550 C. when quenching is effected.

9. A process as claimed in claim 6 wherein said quenching is effected byspraying water on the steel member at a pressure of 3 to 5 atmospheres.

10. A process as claimed in claim 9 wherein the water is sprayed in anamount between 6 to 15 liters per kg. steel.

11. A process as claimed in claim 1 comprising eifecting a slight colddeformation after quenching.

12. A process as claimed in claim 1 comprising heat treating said memberfor 20 to 30 minutes at a temperature between 100 and 380 C., after saidquenching to increase the elastic limit.

13. A process as claimed in claim 3 wherein the silicon content is amaximum of 0.5% and the manganese content is a maximum of 0.8%

14. A process as claimed in claim 1 wherein quenching is effected withthe core at a temperature in the upper range of crystal recovery, andwith the temperature of the shell region above Ar whereby to obtainincreased elongation properties while at least maintaining the strengthproperties.

15. A process as claimed in claim 14 wherein the temperature of theshell region is below the crystal recovery range after quenching.

16. A process as claimed in claim 1 wherein temperature of the coreincreases after the heating has stopped while the temperature of theshell region decreases and drops below the temperature of the core, saidquenching being effected after the temperature of the core has reachedthe temperature of the shell region.

17. A process as claimed in claim 1 wherein said quenching of the memberis effected with water jets.

10 18. A process as claimed in claim 1 wherein said steel member iscircular and has a diameter of 4 to 16 mm.

19. A process as claimed in claim 16, wherein a very thick shell isheated to a temperature which is only 50 to 200 C. from the temperatureat the time when the quenching is started.

References Cited UNITED STATES PATENTS 2,598,694 6/1952 Herbenar 1481502,393,363 1/1946 Gold 61; a1. 148150 1,946,876 2/1934 Northrup 148150RICHARD O. DEAN, Primary Examiner US. Cl. X.R.

